In enhanced mobility MOSFET device applications thick, relaxed Si1-xGex buffer layers have been used as virtual substrates for thin strained silicon layers to increase carrier mobility for both NMOS, Rim et al., Strained Silicon NMOSFETs for high performance CMOS technology, 2001 Symposium on VLSI Technology Digest of Technical Papers, p. 59, IEEE (2001), and PMOS, Nayak et al., High-mobility Strained-Silicon PMOSFETs, IEEE Transactions on Electron Devices, Vol. 43, 1709 (1996). Compared with bulk silicon devices, enhancement in electron mobility of 70% for devices with Leff<70 nm have been reported. Enhancements of up to 40% in high-field hole mobility for long-channel devices have also been found. The main current technique to produce a high quality relaxed Si-xGex buffer layer is the growth of a several micron thick compositionally graded layer, Rim and Nayak, supra. However, the density of threading dislocations is still high, e.g., >106cm−2. In addition, the integration of a several micron thick Si1-xGex layer into device fabrication is not practical.
Recently, alternative methods to efficiently relax strained SiGe layers on silicon have been sought. Based on the SmartCut™ process, Weldon et al., On the mechanism of the hydrogen-induced exfoliation of silicon, J. Vac. Sci. Technol. B. 15, 1065, (1997), for the fabrication of high-quality silicon-on-insulator (SOI) wafers, atomic hydrogen (H+) implantation, followed by an appropriate anneal, has been used to increase the degree of SiGe relaxation and to reduce the density of threading dislocations: Mantl et al., Strain relaxation of epitaxial SiGe layers on Silicon(100) improved by hydrogen implantation, Nuclear Instruments and Methods in Physics Research B 147, 29, (1999); and U.S. Pat. No. 6,464,780 B1, granted Oct. 15, 2002, for Method for the production of a monocrystalline layer on a substrate with a non-adapted lattice and component containing one or several such layers; Trinkaus et al., Strain relaxation mechanism for hydrogen-implanted Si1-xGex/Silicon(100) heterostructures, Appl. Phys. Lett., 76, 3552, (2000); and U.S. patent Publication 2003/0143783 A1, of Maa et al., published Jul. 31, 2003, for Method to Form Relaxed SiGe Layer with High Ge Content. Previously, the implantation of either H+ or H2+ alone, or with boron, He, silicon, or other species for the purpose of relaxing strained SiGe films deposited epitaxially on silicon substrates has been described, U.S. Pat. No. 6,562,703, granted May 13, 2003, to Maa et al., for Molecular Hydrogen Implantation Method for Forming a Relaxed Silicon Germanium Layer with High Germanium Content. To produce the desired relaxation after ion implantation, the wafers have been typically annealed at some high temperature, e.g., at about 800° C. for several minutes.
The method of the SmartCut™ process has been the focus of a number of studies. Cerofolini, et al., Hydrogen-related complexes as the stressing species in high-fluence, hydrogen-implanted, single-crystal silicon, Physical Review B, vol. 46, p. 2061 (1992), used channeling Rutherford backscattering spectrometry (RBS) to measure the silicon atoms displaced from their crystallographic positions, called the “displacement field,” as a result of H2+ ion implantation and subsequent anneals of Silicon (100) wafers. They implanted 0.8×10016 cm−2 of H2+ at 31 keV, which is equivalent to 1.6×1016 cm−2 of H+ ions at 15.5 keV. A very interesting “reverse annealing” effect was observed, i.e., the displacement field increased with temperature for samples annealed for two hours as the temperature increased from 200° C. to 400° C., and reached a maximum at between 350° C. to 400° C., then monotonically decreased for samples annealed at higher temperatures up to 800° C. Also, at a fixed temperature of 200° C., the displacement field increased with annealing time up to at least 400 minutes. Furthermore, it was observed that the implanted hydrogen mostly remained in the samples for anneals up to 400° C., but rapidly escaped from those wafers annealed at higher temperatures. From these and other results, Cerofolini et al. concluded that large pressures from hydrogen complexes, most likely one to two H2 molecules in a silicon vacancy position, were responsible for the observed displacement field.
Frabboni and Gambetti later developed a transmission electron microscopy (TEM) technique to analyze static disorder from similar hydrogen-implanted and annealed silicon wafers, Frabboni et al., Static disorder depth profile in ion implanted materials by means of large angle convergent beam electron diffraction, Physical Review Letters, vol. 81, 3155 (1998). They confirmed the “reverse annealing” effect and found that the peak static disorder for a sample annealed at 300° C. for two hours was more than twice the as-implanted value, and about twice that for a sample annealed at 500° C. for two hours. Recently Frabboni refined this technique and further confirmed these results. Frabboni, Lattice strain and static disorder in hydrogen-implanted and annealed single-crystal silicon as determined by large-angle convergent-beam electron diffraction, Physical Review B, vol. 65, 165436 (2002).